I want to determine crop water requirement for coffee plant s under different shade cover and I want to understand the effects of shade cover on crop water requires
Hi Actual crop water use is directly related to ET. The crop's water use can be determined by multiplying the calculated ET by a crop coefficient, Kc, which takes into account the difference in ET between the crop and reference evapotranspiration.
Article New Technique for Evaluation of Crop Water Requirement
Water requirement is determined in two steps when using climatic data. the first step is to use climate data as input in the most famous equation to calculate reference evapotranspiration (ET0) (Penman-Monteith equation). the second step is to calculate actual crop water requirement (ETc) by multiplying ET0 with crop coefficient. However, since your crop is grown under shade, and solar radiation is a main component in Penman-Monteith equation, you must reduce the net radiation input value by the percentage of shade in the first step. to be more precise, you must measure net solar radiation (nRs) under shade using a reliable sensor.
On the other hand, since the method of shading may affect all you climate parameters (temperature, wind speed, humidity), I strongly recommend you to use a different method. probably using soil moisture monitoring sensor would be a good way.
I think try adjusting the ETo to a lower value. ETo is calculated from the solar radiation so you can adjust that. If you don't have solar, then you can reduce the temperatures as they affect the ETo.